null.h

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/***********************************************************************
 Copyright (c) 1998 by Kevin Atkinson, (c) 1999-2001 by MySQL AB, and
 (c) 2004-2008 by Educational Technology Resources, Inc.  Others may
 also hold copyrights on code in this file.  See the CREDITS file in
 the top directory of the distribution for details.

 This file is part of MySQL++.

 MySQL++ is free software; you can redistribute it and/or modify it
 under the terms of the GNU Lesser General Public License as published
 by the Free Software Foundation; either version 2.1 of the License, or
 (at your option) any later version.

 MySQL++ is distributed in the hope that it will be useful, but WITHOUT
 ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
 License for more details.

 You should have received a copy of the GNU Lesser General Public
 License along with MySQL++; if not, write to the Free Software
 Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301
 USA
***********************************************************************/

#ifndef MYSQLPP_NULL_H
#define MYSQLPP_NULL_H

#include "exceptions.h"

#include <iostream>
#include <string>

namespace mysqlpp {

extern const std::string null_str;      


class MYSQLPP_EXPORT null_type
{
protected:
#if !defined(DOXYGEN_IGNORE)
// Doxygen will not generate documentation for this section.
        template <typename CannotConvertNullToAnyOtherDataType>
        operator CannotConvertNullToAnyOtherDataType() const
        {
                return CannotConvertNullToAnyOtherDataType();
        }
#endif // !defined(DOXYGEN_IGNORE)
};

const null_type null = null_type();


struct NullIsNull
{
#if !defined(DOXYGEN_IGNORE)
// Doxygen will not generate documentation for this section.
        static null_type null_is() { return null; }

        static std::ostream& null_ostr(std::ostream& o)
        {
                o << "(NULL)";
                return o;
        }
#endif // !defined(DOXYGEN_IGNORE)
};


struct NullIsZero
{
#if !defined(DOXYGEN_IGNORE)
// Doxygen will not generate documentation for this section.
        static int null_is() { return 0; }
        
        static std::ostream& null_ostr(std::ostream& o)
        {
                o << 0;
                return o;
        }
#endif // !defined(DOXYGEN_IGNORE)
};

struct NullIsBlank
{
#if !defined(DOXYGEN_IGNORE)
// Doxygen will not generate documentation for this section.
        static const char *null_is() { return ""; }
        
        static std::ostream& null_ostr(std::ostream& o)
        {
                o << "";
                return o;
        }
#endif // !defined(DOXYGEN_IGNORE)
};


template <class Type, class Behavior = NullIsNull> class Null
{
public:
        Type data;
        
        bool is_null;

        typedef Type value_type;

        Null() :
        is_null(false)
        {
        }

        Null(const Type& x) :
        data(x),
        is_null(false)
        {
        }

        Null(const null_type&) :
        is_null(true)
        {
        }

        operator Type&()
        {
                if (is_null)
                        return data = Behavior::null_is();
                else
                        return data;
        }

        Null& operator =(const Type& x)
        {
                data = x;
                is_null = false;
                return *this;
        }

        Null& operator =(const null_type& n)
        {
                is_null = true;
                return *this;
        }

        bool operator ==(const Null<Type>& rhs) const
        {
                if (is_null && rhs.is_null) {
                        return true;
                }
                else if (is_null != rhs.is_null) {
                        return false;   // one null, the other not
                }
                else {
                        return data == rhs.data;
                }
        }

        bool operator ==(const null_type&) const { return is_null; }

        bool operator !=(const Null<Type>& rhs) const
                        { return !(*this == rhs); }

        bool operator !=(const null_type& rhs) const
                        { return !(*this == rhs); }

        bool operator <(const Null<Type>& rhs) const
        {
                if (is_null && rhs.is_null) {
                        return false;
                }
                else if (is_null && !rhs.is_null) {
                        return true;
                }
                else {
                        return data < rhs.data;
                }
        }

        bool operator <(const null_type&) const { return false; }
};


#if !defined(DOXYGEN_IGNORE)
// Doxygen will not generate documentation for this section.

// Specialization the Null template for \c void
template <> class Null<void>
{
public:
        bool is_null;
        typedef void value_type;

        Null() :
        is_null(false)
        {
        }
        
        Null(const null_type&) :
        is_null(true)
        {
        }

        Null& operator =(const null_type&)
        {
                is_null = true;
                return *this;
        }
};

#endif // !defined(DOXYGEN_IGNORE)


template <class Type, class Behavior>
inline std::ostream& operator <<(std::ostream& o,
                const Null<Type, Behavior>& n)
{
        if (n.is_null)
                return Behavior::null_ostr(o);
        else
                return o << n.data;
}

} // end namespace mysqlpp

#endif

---